Highly elastic rubber has been used as a rubber composition used in a carcass member, a stiffener (bead filler) and the like of a tire. As a method of making rubber highly elastic, a method of increasing an amount of a filler, such as carbon black (see, for example, Patent Document 1), a method of increasing crosslinking point by increasing an amount of sulfur of a vulcanizing agent, and the like have been known. However, the method of increasing an amount of a filler, such as carbon black, has problems that the rubber composition may be deteriorated in factory workability and breakage resistance, such as breaking elongation, and the rubber composition may be deteriorated in exothermic characteristics. The method of increasing an amount of sulfur of a vulcanizing agent has problems that the breaking elongation may be deteriorated, and the property change due to thermal degradation may be increased.
In view of the problems, a method of enhancing the elasticity of a rubber composition while suppressing the breaking elongation thereof from being lowered. Examples of the method include a method of adding, to a rubber composition, a non-modified novolak type phenol resin, which is obtained through condensation reaction of a phenolic compound and an aldehyde compound in the presence of an acidic catalyst, and a curing agent. Examples thereof also include a method of adding, to a rubber composition, a modified novolak type phenol resin, which is obtained by modifying a novolak type phenol resin with an unsaturated oil, such as tall oil and cashew oil, or an aromatic hydrocarbon, such as xylene and mesitylene, and a curing agent. The curing agent used may be hexamethylenetetramine (see, for example, Patent Documents 2 and 3).
As a method of enhancing a breaking elongation of a rubber composition while enhancing hardness thereof, such a method for manufacturing a rubber composition has been proposed that contains a step of preparing a wet master batch by dissolving diene rubber in a solvent, and adding a phenol resin and carbon black thereto, followed by agitating (see, for example, Patent Document 4).
However, these methods are still insufficient for enhancing the elasticity of the rubber composition while suppressing the breaking elongation thereof from being lowered.